There is frequently a requirement to detect the onset of arcing from an electrosurgical instrument, either in order to control the operation of the instrument, or because such arcing is considered undesirable in the circumstances. Examples of arc detection systems are U.S. Pat. No. 4,114,623, U.S. Pat. No. 5,976,128, U.S. Pat. No. 4,209,018, U.S. Pat. No. 4,860,745 and U.S. Pat. No. 5,108,391. Most of the above patents disclose the monitoring of the energy related to the non-zero harmonic frequencies of the operating radio frequency signal in order to detect arcing in fixed frequency monopolar systems. The present invention attempts to provide an improved type of arc detection system, without some of the limitations of such prior art systems.